Coating of workpieces by vapor deposition

ABSTRACT

To replenish a bath of molten metal composition in a vacuum chamber, used for the vapor-deposition coating of a workpiece, a wire of the same composition as the bath is continuously fed into the chamber with immersion of its free end into the bath which is maintained at vaporization temperature by an electron beam trained thereon. The wire is preheated, at a point immediately adjoining its immersed end, by the passage of a current therethrough, by electromagnetic induction or by another electron beam trained thereon.

United States Patent [1 1 Donckel et al. I

[ COATING OF WORKPIECES BY VAPOR DEPOSITION [75] Inventors: Georges Henri Donckel, Dudelange,

Luxembourg; Dominique Thomas Francois Leon Joseph Marie Streel, Cointe-Sc1essin, Belgium [731 Assignees: Cockerill-Ougree-Providence S.A.,

Seraing, Belgium; Acieries Reunies De Burhach-Eich-Dudelange S.A., Luxembourg, Luxembourg [22] Filed: Feb. 6, 1973 [21] Appl. No.: 330,126

[30] Foreign Application Priority Data Feb. 8, 1972 Luxembourg 64748 [52] US. Cl 117/107, 117/107.1, 118/48, 118/49, 118/491, 118/495 [51] Int. Cl. C236 13/02 [58] Field of Search 117/107, 107.1; 118/49.5, 118/491, 48, 49

[56] References Cited UNITED STATES PATENTS 2,153,786 4/1939 Alexander et a1. 117/107 X 1 Jan. 14,1975

2,899,528 8/1959 Reichelt 117/107.1 X 3,467,058 9/1969 Box et a1 l17/107.1 X 3,562,002 2/1971 X 3,607,222 9/1971 Kennedy 117/107 X Primary Examiner-Mayer Weinblatt Assistant Examiner-Harris A. Pitlick Attorney, Agent, or Firm-Kar1 F. Ross; Herbert Dubno [57] ABSTRACT To replenish a bath of molten metal composition in a vacuum chamber, used for the vapor-deposition coating of a workpiece, a wire of the same composition as the bath is continuously fed into the chamber with immersion of its free end into the bath which is maintained at vaporization temperature by an electron beam trained thereon. The wire is preheated, at a point immediately adjoining its immersed end, by the passage of a current therethrough, by electromagnetic induction or by another electron beam trained thereon.

5 Claims, 3 Drawing Figures AIR Locx COATING OF WORKPIECES BY VAPOR DEPOSITION FIELD OF THE INVENTION Our present invention relates to a method of and a system for coating sheet-metal strips and other workpieces, by vapor deposition in a vacuum, to form surface layers of thicknesses generally ranging between several tenths and several tens of microns.

BACKGROUND OF THE INVENTION In such a system it is known to maintain a bath of a molten metallic composition in a crucible within a vacuum chamber which may have been evacuated to a pressure of about to 10 Torr. The metallic composition, which may have a single major constituent or may be an alloy of two or more components, is heated to an elevated temperature sufficient to vaporize part of the bath for deposition on the workpiece surface.

In principle, it is possible to work with a limited supply of material, by filling the crucible once and exhausting its contents, or to maintain an indefinite supply by continuously (or in recurrent small steps) replenishing the vapor losses of the bath. The discontinuous or batch method is suitable mainly for the coating of relatively small and/or delicate articles, e.g., for the metallization of printed electronic circuits or for the application of thin metal films to optical elements; these discontinuous coating operations result in uniform and homogeneous deposits, without rough spots, even with relatively high-melting metals. The second method involves the feeding of an elongate solid element of the desired composition, usually a metal wire with a diameter of a few millimeters, into the chamber for progressive immersion of the free end of the element into the bath. This latter technique is generally preferred for the vapor coating of articles of great or indefinite length, such as strips of sheet steel or the like.

The progressive immersion of a wire or other preferably flexible metallic elements into the hot bath creates a certain lack of homogeneity in the melt which in turn leads to nonuniform deposits with excrescences and other rough spots, particularly if the metallic composition has a melting point above l,200C and if the rate of vaporization is high. Thus, whereas a low-melting metal such as aluminum (melting point 660C, vaporization temperature about l,200C) yields satisfactory coatings, the results are less acceptable with highmelting metals such as nickel (melting point 1,455C) and especially alloys thereof, e.g., nickel/chromium (melting point upward of 1,455C, vaporization point upward of 1,800C, according to the composition). This is due to the delay with which the free end of the wire, upon touching the bath surface, is raised to the temperature of the melt from a level near or at room temperature with which it is fed into the vacuum chamber through a suitable air lock. One can observe in such cases the formation of large blobs of molten metal around the immersion point whereby the bath becomes unstable and may even be in danger of explosion, owing to the introduction of occluded gas bubbles from the supply wire into the melt.

Various means have been proposed for remedying these inconveniences, such as the provision of a baffle screening the zone of immersion from the remainder of the bath as disclosed in U.S. Pat. No. 3,467,058. Other prior proposals involve the introduction of certain additives into the bath, e.g., as described in British Pat.

Nos. 1,154,959, 1,246,077 and 1,l62,410.

OBJECT OF THE INVENTION SUMMARY OF THE INVENTION We realize the aforestated object, in conformity with the present invention, by preheating the oncoming'metallic element (referred to hereinafter, for convenience, as a wire) within the vacuum chamber at a location close to its point of immersion. This preheating step should not cause any premature fusion of the element, and should therefore be limited to a temperature well below the melting point of the composition, but should nevertheless be sufficient to accelerate the blending of the freshly supplied material into the melt and to facilitate the release of gas occlusions which are generally present in commercially available wires of nickel, nickel alloys and the like. The immersion of the preheated solid material into the bath prevents the formation of a zone of pasty consistency in the vicinity of the immersion point and homogenizes the metallic melt throughout the bath.

According to a more particular feature of our invention, applicable to metallic compositions with melting points above l,200C, the wire is preheated to a tem perature level which falls short of the bath temperature by not more than 1,000C, preferably by less than 800C.

The preheating of the wire may be carried out in a variety of ways known per se, advantageously electrically. Thus, an electric heating current may be passed through the wire with the aid of a pair of electrodes contacting same, e.g., through the intermediary of a conductive guide roller acting as one of the electrodes. Another, contactless way of generating such a heating current is by electromagnetic induction from a highfrequency source.

In a particularly advantageous embodiment, taking advanntage of the presence of a hard vacuum in the chamber surrounding the bath, an electron gun is utilized as the preheating means. The maintenance of the elevated bath temperature, with the aid of a heat source independent of the one used for preheating the wire, may also be achieved by electronic bombardment.

BRIEF DESCRIPTION OF THE DRAWING The above and other features of our invention will now be described with reference to the accompanying drawing in which:

FIG. 1 is a diagrammatic view of a system for the vapondeposition coating of workpieces in accordance with our invention; and

FIGS. 2 and 3 are fragmentary diagrammatic views showing partial modifications of the system of FIG. 1.

SPECIFIC DESCRIPTION In FIG. 1 we have shown a base and a bell jar 11 forming a closed chamber 3 in which a hard vacuum is maintained by an exhaust pump 12. A workpiece 13, such as a strip of sheet steel, is passed through air locks 14, 15 in the chamber wall so as to be supported in a position in which both surfaces thereof are freely accessible to metallic vapors developed inside the chamber. A refractory crucible 7 on base 10 contains a bath 16 of a metallic composition, e.g., nickel/chromium alloy, which is to be vapor-deposited on the workpiece 13 while the latter travels slowly through the chamber as indicated by an arrow A. Another air lock 17 forms an inlet for a wire 2 which is continuously uncoiled from a reel 1 and is advanced by a pair of driven feed rollers 18, 19. Wire 2 is guided by several rollers 20, 21, 22, 23 so that its free end dips into the bath 16 which is held in a molten state by electron bombardment from an electron gun 8. A source of direct current 4 has one of its terminals (here positive) connected to a sliding contact 24 inside the chamber 3, this contact engaging the advancing wire 2 just ahead of its point of immersion into the bath through the exposed surface of the latter. Current from contact 24 passes longitudinally through the wire into guide roller 22 which is grounded at 6, the negative terminal of source 4 being also connected to ground. This current flow preheats the wire 2 to preferably within 800C of the bath temperature to keep the melt 16 homogeneous and stable. The rate of wire feed, with the aid of a non-illustrated electric motor driving the rollers 18 and 19, may be remotecontrolled by an operator or varied automatically to let the supply of nickel/chromium alloy keep step with the depletion of the bath by evaporation. Heat sources 4 and 8 may be thermostatically controlled, in a manner known per se and not illustrated, to maintain the desired temperature levels. Contacts 22 and 24 may be adjustably mounted to slide along the path of wire 2 for establishing an optimum distance from the bath surface, as determined experimentally.

In FIG. 2 we have shown a modified system in which a source 4' of high-frequency alternating current is connected across an electromagnetic coil 24 to induce a heating current in the wire.

According to FIG. 3, a second electron gun 4" is trained upon the wire 2 to bombard same at a location just ahead of its immersion point, thereby preheating it for the purpose set forth.

EXAMPLE A nickel/chromium alloy of composition 80:20 is to be vapor-deposited on a workpiece 13 of sheet steel in a vacuum of 10 Torr. The crucible 7 has a capacity of 8 kg and a vaporization surface of 350 cm At an operating temperature of 1,800C, the bath emits metal vapors at a rate of about 20 gr/min; fresh alloy is supplied at the same rate by the continuously advancing wire 2 of like composition. The wire, entering the air lock 17 at an ambient temperature of 25C, has a diameter of 3 mm.

A length of wire of 10 cm, extending between contact 24 and grounded roller 22, is brought to red heat (upward of 800C) by the passage of a current therethrough as illustrated in FIG. 1; the heating current varies between 1 l5 and 180 amps for feed rates ranging between 10 and 25 gr/min. We have found that even higher feed rates, on the order of 30 gr/min, may be maintained in our present system without the creation of instabilities in the bath and with formation of a smooth and uniform deposit on the workpiece surface. On the other hand, if the preheating step is omitted, rough spots appear as soon as the feed rate exceeds 8 gr/min.

Naturally, our invention can also be used with other compositions and is susceptible to various structural modifications without departing from the spirit and scope of the appended claims.

We claim:

1. A method of coating workpieces by vapor deposition, comprising the steps of:

disposing a workpiece in an evacuated enclosure; heating a bath of molten metallic composition with a melting point above 1,200C by a first heat source in said enclosure to an elevated temperature suffi-- cient to vaporize part of said composition for deposition on the workpiece;

introducing an elongate solid element of said composition into said enclosure, with progressive immersion of a free end of the element into the bath through an exposed surface thereof to replenish vapor losses; and

preheating said element by a second heat source independent of said first heat source within said enclosure, at a location spaced from its point of immersion, to a level not more than l,000C lower than said elevated temperature.

2. A method as defined in claim 1 wherein said element is preheated by the passage of a heating current therethrough.

3. A method as defined in claim 2 wherein said heating current is generated in said element by electromagnetic induction.

4. A method as defined in claim 1 wherein said element is preheated by electronic bombardment.

5. A method as defined in claim 1 wherein the bath is maintained at said elevated temperature by electronic bombardment.

* IIK 

1. A METHOD OF COATING WORKPIECES BY VAPOR DEPOSITION, COMPRISING THE STEPS OF: DISPOSING A WORKPIECE IN AN EVACUATED ENCLOSURE; HEATING A BATH OF MOLTEN METALLIC COMPOSITION WITH A MELTING POINT ABOVE 1,200*C BY A FIRST HEAT SOURCE IN SAID ENCLOSURE TO AN ELEVATED TEMPERATURE SUFFICIENT TO VAPORIZE PART OF SAID COMPOSITION FOR DEPOSITION ON THE WORKPIECE;
 2. A method as defined in claim 1 wherein said element is preheated by the passage of a heating current therethrough.
 3. A method as defined in claim 2 wherein said heating current is generated in said element by electromagnetic induction.
 4. A method as defined in claim 1 wherein said element is preheated by electronic bombardment.
 5. A method as defined in claim 1 wherein the bath is maintained at said elevated temperature by electronic bombardment. 